The invention relates to a device and a method for operating a linear lambda probe of an internal combustion engine.
A linear lambda probe is used to determine the oxygen concentration in the exhaust gas of an internal combustion engine. It has two pairs of electrodes and a measuring chamber which is connected to the exhaust gas stream via a diffusion barrier. The first pair of electrodes (measuring electrodes) are arranged between the measuring chamber and air and are used to measure the oxygen concentration in the measuring chamber. The second pair of electrodes (pump electrodes) are arranged between the measuring chamber and the exhaust gas stream. This permits oxygen ions to be pumped out of the measuring chamber, or into it, when a current Ip with a corresponding polarity is applied.
In this way it is possible to generate a dynamic equilibrium between the flow of oxygen through the diffusion barrier and the flow of oxygen ions through the pair of pump electrodes. The oxygen concentration in the measuring cell which can be determined using the measuring electrodes is suitable here as a regulating criterion. A preferred value is, for example, 450 mV for xcex=1. The pump current Ip which flows in this case is a measure of the oxygen concentration in the exhaust gas (and also for xcex after numerical conversion).
Hitherto, the application of the linear lambda probe has been restricted to the nonsupercharged, stoichiometric mode of operation (Pa=1 bar, xcex=1) of the engine. Correspondingly, only small pump currents are necessary to maintain the equilibrium (xcex=1) in the measuring cell (|IP| less than xcx9c2.5 mA).
For lean engines, an operating mode up to xcex=4 is provided, which requires a drastically increased pump current Ip. When operating in a supercharged (turbo) engine, an exhaust gas pressure of up to 2 bar is produced. The pressure sensitivity of the probe leads to a further increase in the maximum necessary pump current of up to xc2x112 mA.
The dynamic resistance of the diffusion barrier has a temperature dependence which leads to errors in the transmission ratio. This is countered by measuring the probe temperature and regulating it by means of a heating element which is installed in the probe. For reasons of cost, a separate thermal element is dispensed with here. Instead, the highly temperature-dependent internal resistance Ris of the probe is measured.
A customary measuring method for determining the internal resistance Ris of the probe is to apply to the probe terminal Vs+ a measuring signal M formed from a square wave alternating currentxe2x80x94for example 500 xcexcAss (peak-to-peak) with a frequency fm of, for example, 3 kHz. This alternating current brings about an alternating voltage of 500 xcexcAss*100 xcexa9=50 mVss with an internal resistance Ris of the probe of 100 xcexa9, for example. This alternating voltage is amplified in an amplifier, for example by the factor ten, and then rectified. The direct voltage Vri which has been produced in this way can be used and further processed as a measure of the probe temperature.
A known evaluation circuit is illustrated in FIG. 1 and will be described in more detail below.
This circuit has certain disadvantages:
When the evaluation circuit is supplied with a supply voltage Vcc=5 V which is generally already available, a center voltage Vm of approximately 2.5 V is produced. The voltage chain which is present at the probe is obtained as:
Vm less than |Rc*Ip+Vp|+Vsat; where 
Rc=30 to 100 xcexa9=overall calibration resistance (manufacturer-dependent),
Vp=xe2x88x92350 to +450 mV; polarization voltage of the pump cell,
Vsat=100 to 200 mV; saturation voltage of the pump current source P; this limits the maximum possible pump current Ip to  less than 10 mA, and therefore does not correspond to the requirements (Ip=xc2x112 mA);
a common mode signal (Vmxc2x12 V) is superimposed on the pump current Ip. The measurement is falsified by up to xc2x10.3%% by the finite common mode expression of real integrated amplifiers (for example 65 dB);
in addition, the polarization voltage of the pump cell (xe2x88x92350 mV when xcex less than 1) results in a zero crossover point error xcex94Ip of approximately 5 xcexcA. As the pump current Ip is the primary measurement signal of the oxygen probe, these errors are included directly in the overall precision of the pump current Ip. This limits the precision of lambda control and thus constitutes a significant problem;
a further fundamental problem of this circuit arrangement is the reciprocal influence between the Nernst voltage Vs and the square wave voltage Vr which is produced from the measurement of the internal resistance. This square wave voltage Vr also appears at the input of the controller R and thus constitutes a control error. The controller will attemptxe2x80x94within the scope of its bandwidthxe2x80x94to compensate this control error. To do this, it changes the pump current Ip, which in turn has effects on Vs. As the pump current Ip is the measurement variable for xcex, the primary probe signal Vs is falsified. In turn the change in Vs is superimposed on the square wave voltage Vr. The effect of this is to cause the signal roof of the square voltage Vr to slope, thus bringing about a considerable amplitude error during the rectification;
when EMC interference signals occur there is also a considerable deviation of the actual measured value of the internal resistance Ris.
The object of the invention is to specify a method for operating a linear lambda probe which makes available values of the pump current Ip which correspond to the requirements set, said method avoiding the described common mode error and significantly improving the precision of the measurement of the pump current so that there is no reciprocal influence between the Nernst voltage Vs and the square wave voltage Vr, and the precision of the measurement of the internal resistance is improved, and which lambda probe remains operational even at a low battery voltage (Vb less than or equal to +6 V). The object of the invention is also to specify a device for carrying out this method.
This object can be achieved according to the invention by a method for operating a linear lambda probe having a first terminal, a second terminal, a third terminal and a fourth terminal, comprising the steps of:
generating a current with a square-wave profile and relatively low frequency from an oscillator signal with a frequency,
supplying the current at the first terminal as a measurement signal,
tapping a sum voltage between the first and second terminals, whose upper and lower envelopes determine an upper value and a lower value,
referring the sum voltage to the difference of a predefined center voltage and a predefined reference voltage, and
forming the mean value corresponding to the difference between a Nernst voltage and reference voltage from the upper value and lower value of the sum voltage and
converting the mean value into a proportional pump current which brings about, at the calibration resistor of the lambda probe, a voltage drop which is used as a measure of the oxygen concentration.
The step of generating the current can be performed by means of frequency division.
Another method for operating a linear lambda probe of an internal combustion engine having a first terminal, a second terminal, a third terminal and a fourth terminal, comprises the steps of:
a current with a square-wave profile and relatively low frequency which is derived from an oscillator signal with a frequency by means of frequency division is supplied at the first terminal as a measurement signal which brings about a square-wave voltage which drops across the internal resistor of the probe and forms, with the Nernst voltage which can be tapped between the first and second terminals, a sum voltage whose upper and lower envelopes determine an upper value and a lower value,
the sum voltage is referred to the difference of a predefined center voltage and a predefined reference voltage, and
in order to determine the oxygen concentration in the exhaust gas of the internal combustion engine, the mean value corresponding to the difference between the Nernst voltage and reference voltage is formed from the upper value and lower value of the sum voltage and is converted into a proportional pump current which brings about, at the calibration resistor of the lambda probe, a voltage drop which is used as a measure of the oxygen concentration in the exhaust gas of the internal combustion engine.
In both methods, the difference between the upper value and lower value of the sum voltage can be formed and can be used as a temperature measuring voltage for regulating the temperature of the probe. The temperature measuring voltage can be low-pass filtered. A predefined oxygen reference current can be fed to the reference cell of the lambda probe.
An embodiment according to the present invention can be a device for operating a linear lambda probe of an internal combustion engine, comprising:
an evaluation circuit which is connected to the lambda probe via its terminals,
an oscillator which generates an oscillator signal having a frequency, and a measurement signal which is derived therefrom by means of frequency division and has a relatively low frequency which is fed to the first probe terminal,
a difference amplifier whose inverting input is connected to the third probe terminal and whose noninverting input is connected to the fourth probe terminal,
a controller and a pump current source,
a second capacitor in which the lower value of the sum voltage is continuously stored is provided,
a third capacitor in which the upper value of the sum voltage is continuously stored is provided, the base points of the second and third capacitors being capable of being connected to the second probe terminal and being connected to the negative pole of a reference voltage, the positive pole of the reference voltage being connected to the positive pole of the center voltage,
a decoupling amplifier is connected downstream of each of the other terminals of the second and third capacitors, wherein
the outputs of the two decoupling amplifiers are connected to one another by means of a voltage divider whose tap is connected to the noninverting input of the controller,
whose output is connected to the fourth probe terminal, and
whose inverting input lies at the center voltage and is connected to the noninverting input of the pump current source,
the inverting input of the pump current source is connected to the third probe terminal, and
the output of the pump current source is connected to the second probe terminal.
A difference amplifier can be provided whose noninverting input is connected to the output of the first decoupling amplifier, whose inverting input is connected to the output of the second decoupling amplifier, and at whose output the temperature measuring voltage can be tapped. A low-pass filter can be connected downstream of the difference amplifier. A first capacitor whose base point can be connected to the second probe terminal via a switch, and to the base points of the second capacitor and third capacitor via a further switch, and whose other terminal is connected to the first probe terminal via a switch, to the other terminal of the second capacitor via a switch, and to the other terminal of the third capacitor via a switch. A switch can be provided via which a predefined oxygen reference current can be fed to the second capacitor, and to the reference cell of the lambda probe via said second capacitor and the first capacitor, as long as the sum voltage is at its upper value, and a switch can be provided via which the predefined oxygen reference current can be fed to the third capacitor, and to the reference cell of the lambda probe via said third capacitor and the first capacitor, as long as the sum voltage is at its lower value. A circuit can be provided for actuating the switches, which circuit alternately connects, with the frequency of the oscillator signal, the first capacitor to the reference cell of the lambda probe via the switches, and to the third capacitor via the switches, as long as the sum voltage is at its upper value, and to the reference cell of the lambda probe via the switches, and to the second capacitor via the switches, as long as the sum voltage is at its lower value.